This invention relates to a catalytic hydrocracking process and a catalyst for use therein. The invention is particularly concerned with an improved process for producing middle distillate products using a catalyst highly selective for such products.
Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel, and other hydrocarbon liquids known as middle distillates as well as lower boiling liquids, such as naphtha and gasoline, by hydrocracking a hydrocarbon feedstock derived from crude oil. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical gas oil comprises a substantial proportion of hydrocarbon components boiling above about 700.degree. F., usually at least about 80 percent by weight boiling above 700.degree. F. A typical heavy gas oil has a boiling point range between about 600.degree. F. and 1050.degree. F.
Hydrocracking is generally accomplished by contacting, in an appropriate reaction vessel, the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a lower overall average boiling point product containing a distribution of hydrocarbon products desired by the refiner. Although the operating conditions within a hydrocracking reactor have some influence on the yield of the products, the hydrocracking catalyst is the prime factor in determining such yields. At the present time, middle distillates are not in high demand relative to gasoline in the United States; however, marketing surveys indicate that there will be an increased demand for middle distillates as the year 2000 approaches. For this reason, refiners have recently been focusing on midbarrel hydrocracking catalysts which selectively produce middle distillate fractions, such as turbine fuel and diesel fuel, that boil in the 300.degree. F. to 700.degree. F. range.
The three main catalytic properties by which the performance of a midbarrel hydrocracking catalyst is evaluated are activity, selectivity, and stability. Activity may be determined by comparing the temperature at which various catalysts must be utilized under otherwise constant hydrocracking conditions with the same feedstock so as to produce a given percentage, normally about 60 percent, of products boiling below 700.degree. F. The lower the activity temperature for a given catalyst, the more active such a catalyst is in relation to a catalyst of higher activity temperature. Selectivity of midbarrel or middle distillate hydrocracking catalysts may be determined during the foregoing described activity test and is measured as the percentage fraction of the 700.degree. F.--product boiling in the desired midbarrel product range, e.g., 300.degree. F. to 700.degree. F. for diesel fuel and 300.degree. F. to 550.degree. F. for turbine fuel. Stability is a measure of how well a catalyst maintains its activity over an extended time period when treating a given hydrocarbon feedstock under the conditions of the activity test. Stability is generally measured in terms of the change in temperature required per day to maintain a 60 percent or other given conversion.
U.S. Pat. Nos. 4,062,809 and 4,419,271, the disclosures of which are hereby incorporated by reference in their entireties, disclose two different types of very effective middle distillate hydrocracking catalysts. The catalyst of U.S. Pat. No. 4,062,809 contains molybdenum and/or tungsten plus nickel and/or cobalt on a support of silica-alumina dispersed in gamma alumina. U.S. Pat. No. 4,419,271 teaches that the catalyst of U.S. Pat. No. 4,062,809 can be improved by adding an aluminosilicate zeolite to the support, thereby producing a catalyst containing molybdenum and/or tungsten and nickel and/or cobalt supported on a mixture of an aluminosilicate zeolite, preferably an ultrahydrophobic zeolite known as LZ-10 zeolite, and a dispersion of silica-alumina in a gamma alumina matrix. The presence of the zeolite in this catalyst increases the activity of the catalyst without significantly affecting the selectivity.
Although the catalysts of the above-discussed patents are highly effective middle distillate hydrocracking catalysts and have proven themselves in commercial environments, there is always a demand for new hydrocracking catalysts with superior overall activity, selectivity, and stability for middle distillate hydrocracking.